ControllerInterface: Don't pass m_devices to the backends

Previously, the devices vector would be passed to all backends. They
would then manually push_back to it to add new devices. This was fine
but caused issues when trying to add synchronisation.

Instead, backends now call AddDevice() to fill m_devices so that it is
not accessible from the outside.

Source/Core/InputCommon/ControllerInterface/Xlib/Xlib.h

@@ -7,13 +7,13 @@
 #include <X11/Xlib.h>
 #include <X11/keysym.h>
 
-#include "InputCommon/ControllerInterface/Device.h"
+#include "InputCommon/ControllerInterface/ControllerInterface.h"
 
 namespace ciface
 {
 namespace Xlib
 {
-void Init(std::vector<Core::Device*>& devices, void* const hwnd);
+void Init(void* const hwnd);
 
 class KeyboardMouse : public Core::Device
 {